1. Field of the Invention
The present invention relates to a process for the hydraulic fracturing of subterranean formations surrounding oil wells and gas wells by means of injection of a fracturing fluid into the well, wherein the fracturing fluid comprises a solution of non-polar organic liquid or oil, a polyampholyte and, possibly, a polar cosolvent and water.
2. Description of the Prior Art
Hydraulic fracturing has been widely used for stimulating the production of crude oil and natural gas from wells completed in low permeability reservoirs. The methods employed normally require the injection of a fracturing fluid containing a suspended propping agent into a well at a rate sufficient to open a fracture in the exposed formation. Continued pumping of fluid into the well at a high rate extends the fracture and leads to the buildup of a bed of propping agent particles between the fracture walls. These particles prevent complete closure of the fracture, as the fluid subsequently leaks off into the adjacent formation, and result in a permeable channel extending from the wellbore into the formation. The conductivity of this channel depends upon the fracture dimensions, the size of the propping agent particles, the particle spacing and the confining pressures. Studies of conventional fracturing operations indicate that fracture widths seldom exceed about one-fourth inch and that conductivities in excess of about 250,000 millidarcy-inches are rarely obtained. The average width and conductivity are considerably lower than these values.
With the advent of declining reserves, the drilling and stimulation of higher temperature wells in increasing the drilling and completion of light dry gas and water sensitive formation is also on the rise. The industry is relying to a greater extent on hydrocarbon fluids to drill and complete these wells which cannot be treated with the normal water-based fluids, therefore, there has been a substantial need for hydrocarbon-based viscosifiers which exhibit good performance at high temperatures.
A desirable formulation, both for drilling fluids and hydraulic fracturing fluids, would be a homogeneous fluid which possesses adequate viscosity of 30-1,000 cps-A non-polar, organic-liquid-based, fluid-containing polymer viscosifiers would meet the above-stated requirements.
Since the beginning of recorded oil well production hydrocarbon-based viscosifiers have played an important role in hydraulic fracturing fluids. Some of these viscosifiers have been either metal soaps of fatty acids, or metal soaps of partially esterified phosphates. Both of these impart viscosity to hydrocarbons, but the metal soaps of fatty acids have inherent thermal thinning properties, which give them limited utility at higher temperatures. The metal soaps of partially esterified phosphates have the disadvantage of being extremely pH sensitive, along with being thermally thinning.
So a viscosifier that has the advantage of maintaining viscosity at high temperatures and/or is not susceptible to variations in pH would represent an advancement over the prior art.
The instant invention differs from a number of applications, U.S. Ser. No. 223,482, U.S. Pat. No. 4,361,658; U.S. Ser. No. 136,837, U.S. Pat. No. 4,322,329, and U.S. Ser. No. 106,027, U.S. Pat. No. 4,282,130, filed by Robert D. Lundberg, one of the instant inventors, et al. These previously-filed applications were directed to the gelling of the organic liquid by a water insoluble, neutralized, sulfonated polymer, whereas the instant invention is directed to fracturing fluids formed from a non-polar, organic liquid and a polyampholyte.
In U.S. Ser. No. 547,955, filed Nov. 2, 1983, abandoned, two polymers are mixed to produce an interpolymer complex which at relatively low concentrations forms a three-dimensional network with a gel-like behavior. Interpolymer complexes are much more effective in forming a network than are single associating polymers, leading to gels of higher strength. The instant polyampholytes are not a polymeric complex as defined in U.S. Ser. No. 547,955.
In U.S. Ser. No. 547,955 the interpolymer complexes in hydrocarbon solutions are obtained by mixing two polymers which are strongly associated with each other. One polymer will contain anionic groups along or pendant to its backbone; and the other polymer will contain cationic groups. The coulombic interaction between cationic and anionic groups leads to network formation, if each chain contains interacting groups in multiple locations.
In the instant invention low charge density polyampholytes are formed in nonaqueous solutions by sulfonation of a preformed base-containing copolymer, such as styrene-4 vinylpyridine. In a polyampholyte the cationic and anionic moieties which are located on the same chain backbone, i.e., sulfonate and 4-vinylpyridine units, interact very strongly with each other. Therefore, these polyampholytes strongly interact with each other, forming a tight networks, i.e., gel, since each chain contains interacting groups in multiple locations. The gel structure, to a first approximation, more tightly bound than its low charge density sulfonate ionomer counterpart and, more importantly, the properties of these gels are improved over the interpolymer complexes. Even more importantly, these polyampholytic gel systems are formed without having to mix oppositely-charged copolymers, as taught in U.S. Ser. No. 547,955.
A solution of polyampholyte and a non-polar organic solvent will have a relatively high viscosity. This can be used as is, if the solution viscosity is not excessive, and, if excessive, the solution viscosity can be modified by the incremental addition of a variety of polar cosolvents, modifying the solution's rheological properties. If by the co-mining water with a solution of polyampholytes and a non-polar solvent, the polar cosolvent is extracted into the water phase, an increase in viscosity or gelling would result in the ampholyte solution during injection into a well. This viscous gel can then be used as a fracturing fluid.